There has been interest in the development of liquid biofuels as these processes have the potential to directly fix carbon dioxide into transportation fuels, which is potentially carbon neutral and politically attractive. Cellulose based biofuels including bioethanol, algae-derived lipids, cyanobacteria, and algae derived hydrogen (H2) are among the most studied biofuels. Despite the promise of these technologies and processes, there are specific limitations that preclude their wide-spread application. For example, post-processing of algal cells and derived lipids imposes higher production costs on algal biodiesel. The production rates of H2 from cyanobacteria still remains low and productivity needs to be improved. Genetically engineered photosynthetic organisms have also been explored for bioethanol production. However separation of ethanol from the aqueous phase remains a challenge.
Microbial fuel cells have been under investigation and development for more than a century, as the use of cells to harvest electrical energy from waste streams is attractive for many reasons. In a biofuel cell, biological catalysts are used on an anode to oxidize biofuels, and a cathode is created that can use the generated electrons to reduce oxygen to water. These systems can either be microbial with living cells on the electrodes, or they can be enzymatic systems, with purified enzymes on the electrodes. In both designs, power can be generated from the oxidation of biofuels, and there are many advantages to these systems over conventional fuel cells and other power generation schemes. However, much research still needs to be done with microbial fuel cells to make them practical and cost-efficient. A significant limitation for both enzymatic and microbial fuel cells is the need for mediators to enable electrical contact between the biological components and inorganic electrode. In some microbial systems, these mediators are made by the organisms themselves, and in other technologies, synthetic mediators are added to the system. In some systems, cells must make physical contact with the electrodes for electron transfer. This can be a significant limitation as it reduces the cellular mass that can be used for biochemical conversion.
Natural gas is frequently purified from “sour gas”, which is a natural gas deposit containing high volumes of sulfur or hydrogen sulfide. The separate sulfur compounds are a significant waste product produced by these processes, lacking an economical or sustainable disposal process.